twenty-something (or twentysomething) is a term used for people at the age of 20 to 29. its is used instead of "twentyfive years old" for example, because at this age its harder to tell how old someone is (by looking at him/her) than it is in the teenage years (and often it "matters" less).

dude #1: how old is Tim?
dude #2: I don't know.. he is a twenty-something, thats for sure
dude #1: thank you for this exact piece of information

A specific demographic group, those aged 20-29. Often Obama supporters (regardless of the country they live in), heavy drinkers, users of urbandictionary.com (and the souce of all the worthwhile-to-read definitions, as opposed to the adolescents that write the unfunny definitions that either profess their love for a gf/bf or proclaim their hatred of a schoolmate), students, servers, bartenders, clubgoers, in massive debt yet still optimistic, and believe that they can change the world if they just work together (until they reach the threshold age of 30, when 98% people realize the futility of their causes).

The author of this UD definition belongs to twenty-somethings demographic.

The courageous protesters at Tiananmen Square were mostly twenty-somethings.